The present invention relates to an electromagnetic flowmeter, and in particular, to an electromagnetic flowmeter in which is efficiency of a magnetic circuit is improved thereby to enable to reduce power consumption.
In a prior art electromagnetic flowmeter, as shown in FIG. 1, two coils C.sub.1, C.sub.2 are provided opposing to each other on the periphery of a pipe P having an insulated inner surface, and an alternating magnetic field B is applied to a fluid F in a diametric direction of the pipe P as shown by the arrow, and a signal voltage e proportional to the product of an intensity of the magnetic field B and a flow velocity of the fluid F is detected by electrodes E.sub.1, E.sub.2.
In this flowmeter, the pipe P which constitutes a circular flow path (having a circular cross section) serves as a gap of the magnetic circuit, and thus, since the magnetic reluctance is large, there is a problem in that a large exciting power is required. Furthermore, since the distribution of weight function within the pipe is not uniform, another problem is involved in that an instrumentation error is caused when the flow of the fluid F becomes irregular due to an elbow and a valve.
In order to eliminate the instrumentation error due to the irregular flow, a particularly designed electromagnetic flowmeter is known from Japanese Patent Laid-Open (Kokai) Publication No. 58-87418 (1983) in which as shown in FIG. 2, in addition to a pair of coils C.sub.a1, C.sub.a2, another pair of coils C.sub.b1, C.sub.b2 orthogonal to the pair of coils C.sub.a1, C.sub.a2 are provided, and a signal voltage ea produced by an alternating magnetic field Ba applied by the pair of coils C.sub.a1, C.sub.a2 is detected by electrodes E.sub.a1, E.sub.a2, and a signal voltage eb produced by an alternating magnetic field Bb applied by the pair of coils C.sub.b1 C.sub.b2 is detected by electrodes E.sub.b1, hd Eb2. In this electromagnetic flowmeter, the pair of electrodes E.sub.a1, E.sub.a2 and the pair of electrodes E.sub.b1, E.sub.b2 are disposed by shifting 90.degree. from each other, and since the distribution of weight functions of both pairs is arranged to complement each other, even when the irregular flow occurs, if a plus error is caused in one of the two pairs of electrodes, a minus error will be caused in the other of the two pairs of electrodes. Accordingly, a true flow rate of the fluid can be calculated by considering a difference between the signal voltages of both the pairs of electrodes. However, the problem of the requirement of the large exciting power still remains.
A proposal for obtaining an electromagnetic flowmeter of small power consumption by improving the efficiency of a magnetic circuit and reducing the exciting power is described in Japanese patent Laid-Open (Kokai) Publication No. 57-200822. In this electromagnetic flowmeter, as shown in FIG. 3, when a current flows through a coil C wound about a core K made of a soft or semi-hard magnetic material, a magnetic field is produced so that the magnetic flux generated in a core K departs from an inner yoke Y.sub.i1 and passes through a fluid F as magnetic flux B.sub.1, and again passes through the fluid F as magnetic flux B.sub.2 from an outer yoke Yo, and returns to the core K through an inner yoke Y.sub.i2. A signal voltage e1 is generated by the B.sub.1 and a signal voltage e.sub.2 is generated by the magnetic flux B.sub.2, and these signal voltages e1, e2 are detected by electrodes E.sub.1, E.sub.2.
In this electromagnetic flowmeter, small power consumption can be achieved because of a small magnetic reluctance of a magnetic gap constituting the fluid path. On the other hand, since a part of the magnetic circuit is formed within the pipe P, there are problems in that it becomes an obstruction to the flow of the fluid, it increases a pressure loss, and it hinders passage of solid materials.